Page:Popular Science Monthly Volume 88.djvu/484

 Recent Radio Inventions

Microphonic Relays; An Unusual Quenched Spark-Gap; a Shpping-Contact Detector

By A. F. Jackson

��FOR a number of years inventors struggled to produce microphonic relays, but their work was practi- cally without substantial success. It was not found possible to build an instrument which would magnetically modulate the current through a microphone contact in such a way that all the vibrations of the human voice could be reproduced and magnified. This, nevertheless, d i d not prevent the de- velopment of mi- crophonic relays that would aug- ment the energy of current having a single definite fre- q u e n c y. Instru- ments of this sort are shown in 1915 U. S. patent No. 1,163,180, issued to W. Schloemilch and A. Leib.

One arrangement of this patent is shown in Fig. 1. The antenna, tun-

���Fig. 1 . With a microphonic relay of this sort, tremendous magnifications may be obtained

��ing and rectifying system a, h, c, f, leads the converted, pulsating energy of the received waves to the first amplifier di. This consists of a wire vibrating system gi connected mechanically to a micro- phone hi. The tension of the vibrating Avire is variable, and is to be adjusted so that its mechanical period is the same as the sound period of the incoming wave groups. Thus the wire is made to vi- brate, through resonance, and a great effect is produced upon the microphone. The current from battery ki \s varied by the first relay and led through the mag- nets and the second-step relay, which controls the current from a second bat- tery. The second step of amplification is carried into the third relay and its output through switch m either into the loud-speaking telephone n or into the delicate contact relay p, the final relay

��01, and the Morse printer o. The relay p is not of the microphonic type, like those of the first three steps, but has a tuned wire pi in contact with a sluggish spring p2.

When signals are received of the group frequency to which all these re- lays are attuned, the third-step relay sends a strong current into the intensi- fying instrument p. The vibrations of the wire pi practically open the local circuit of this last named apparatus and so permit the final re- lay to close and the Morse printer to register. This same microphonic amplifying appara- tus may be applied to sustained-wave reception, if an in- terrupter is insert- ed at either the sender or receiver; in this case, the vi- brating wires are tuned to the inter- rupter frequency. In the same way, beats or heterodyne receivers may be used, and the relays tuned to the resulting signal frequency. With apparatus of this kind, tremendous magnifications of signals may be ob- tained; the microphonic relays must, however, be protected from vibration and kept in accurate adjustment. In place of the intensifying relay p, a trans- former and rectifier may be used to make the amplified alternating currents oper- ate a direct-current relay.

By the combination' of large amplifi- cation from the microphone relays, con- nected in cascade, with exceedingly sharp resonance to tone frequency, some ex- tremely interesting results have been se- cured. Using a single receiving antenna, tuner and detector, it has been found pos- sible to record, on separate Morse tapes, messages from three different transmit-

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